Method for producing electric energy in a biofuel-powered fuel cell

ABSTRACT

A method for producing electric energy in a biofuel-powered fuel cell, the metal in the first acid metallic salt solution forming a redox pair having a normal potential between -0.1 and 0.7 V and the metal in the second acid metallic salt solution forming a redox pair having a normal potential between 0.7 and 1.3 V, both metals preferably being vanadium which forms the redox pairs vanadium(IV)/(III) and vanadium (V)/(IV), respectively; 
     carbohydrate being supplied as fuel to the first reactor (1) and 
     the reaction in the first reactor (1) being effected in the presence of platinum or ruthenium as the first catalyst (2).

The present invention relates to a method for producing electric energyin a biofuel-powered fuel cell.

PRIOR-ART TECHNIQUE

The most efficient technique so far of converting the chemical energy(combustion heat) of a fuel into electric energy has proved to be theeffecting of the conversion in fuel cells. Several types of fuel cellsare known. What essentially distinguishes the different fuel cells isthe electrolyte which is responsible for the current transfer in thecell, as well as the fuel used. The electrolyte may be, for instance,concentrated alkali or concentrated H₃ PO₄. In a conventional fuel cell,the fuel is consumed at the negative pole of the cell, thereby formingoxidised products (products of combustion) and transferring electrons tothe cathode. The oxidising agent necessary for the combustion, in mostcases air or pure oxygen, is converted at the positive pole of the cellsuch that the oxygen molecule is reduced to water or components ofwater, such as hydrogen ions or hydroxide ions, electrons beingtransferred from the anode to the oxidising agent.

The advantage of these fuel cells is that the combustion can be effectedat a low temperature, and that the available free energy change (Gibbs'free energy) is in the theoretical case directly converted into electricenergy, thereby preventing the power losses which occur in processes inwhich differences in temperature are utilised, for example in a Carnotheat engine.

Regarding the fuel for fuel cells, practically nothing but hydrogen gashas been used so far. Fuels such as carbon monoxide or methanol maypossibly also be used, but they have not yet been found to function in asatisfactory manner. Methanol involves the same drawback as when usinghydrogen gas as fuel, i.e. in the production a when using hydrogen gasas fuel, i.e. in the production a considerable amount of carbon dioxideis formed. Hydrogen gas is now produced almost exclusively from naturalgas, and the carbon dioxide formed in this process promotes theso-called greenhouse effect.

Prior-art techniques of utilising the energy content of biofuels havecomprised, for example, direct combustion thereof, e.g. in the form ofwood, and utilisation of the increase in temperature, i.e. the releasedheat. Moreover, the biofuels have been gasified, thereby obtaining amixture of hydrogen gas and carbon monoxide, which may then be burntdirectly in a suitable apparatus or be chemically combined under theaction of a suitable catalyst for forming methanol, methane,paraffin-oil or other carbonaceous fuels. Besides, by a suitablereaction, the biofuel or part thereof can be converted into fermentablesugar, which is subsequently fermented to ethanol. This ethanol may thenbe used in e.g. internal combustion engines, either separately or mixedwith petrol.

It is previously known to use vanadium sulphate or other vanadiumcompounds in electrochemical cells (see M. Skyllas-Kazacos and F.Grossmith, J. Electrochem. Soc. 134 (1987) 2950; M. Skyllas-Kazacos, M.Rychick and R. Robins, U.S. Pat. No. 4,786,567, 188; B. Sun and M.Skyllas-Kazacos, Electrochim. Acta, 36 (1991) 513; and M. Kazacos, M.Cheng and M. Skyllas-Kazacos, J. Appl. Electrochem. 12 (1982) 87). Anelectrically renewable accumulator is then involved, which has beendeveloped by Skylla-Kazacos et al. If vanadium-based fuel cells aretaken into consideration, and if the used fuel is hydrogen gas,reference can be made to the chemically renewable cells which have beendeveloped by Kummer and Oei (see J. T. Kummer and D. G. Oei, J. Appl.Electrochem. 15 (1985) 619; and J. T. Kummer and D. G. Oei, J. Appl.Electrochem. 12 (1982) 87).

It is also known to use glucose as fuel in fuel cells (see J. R. Rao, G.J. Richter, F. von Sturm and E. Weidlich. Bioelectrochem. Bioenerg. 3(1976) 139). These fuel cells are adapted to be implanted in the humanbody for driving a pacemaker. However, the glucose is oxidised togluconic acid only, and only a small amount of the reducing capacity ofthe glucose is utilised. Fuel cells in which polyalcohols (see W. Hertland H. H. Weetall, Bioelectrochem. Bioenerg. 14 (1985) 357; and W. Hertland R. G. Schaeffler, U.S. Pat. No. 4,578,323, 25 Mar. 1986) orcarbohydrates (see W. Hertl and H. H. Weetall, Bioelectrochem. Bioenerg.14 (1985) 367; and W. Hertl and R. G. Schaeffler, U.S. Pat. No.4,578,323, 25 March 1986) are used with e.g. anthraquinone as catalyst,are also known, but they require illumination to produce electriccurrent.

Further, there are prior-art methods of producing electricity from redoxbatteries, for example by means of Ti/Fe (see DOE/NASDA/12 726-24; NASATM-83677; N. H. Hagedorn, "NASA Redox Storage System DevelopmentProject", October 1984; ERDA/NASA 1022/77/10; NASA TM-X-73669; M. A.Reid and R. F. Gahn, "Factors Affecting the Open-Circuit Voltage andElectrode Kinetics of Some Iron/Titanium Redox Flow Cells", 1987; and C.C. Liu, R. T. Galasco and R. F. Savinelli, J. Electrochem. Soc. 126(1979) 357, 128 (1981) 1755; 129 (1982) 2502), orvanadium(V)/vanadium(II) (see M. Skyllas-Kazacos and F. Grossmith, J.Electrochem. Soc. 134 (1987) 2950), but in all these systems,electricity and not a chemical reaction is used for recharging.

BACKGROUND OF THE INVENTION

It is thus not previously known to use carbohydrates as fuel incombination with vanadium in the solution of electrolytes for producingelectric energy in a fuel cell. None of the above-mentioned publicationsdescribes or indicates the use of said combination.

It is a global ambition to find alternative methods of producingelectric energy, including alternative fuels for e.g. vehicle engines.Sooner or later, the fossil fuels of the earth will be depleted, alsoincluding all the uranium accessible, which means that the use ofbiofuels, i.e. fuels based on cultivatable, biological materials, maybecome most interesting. Moreover, a relief of the so-called greenhouseeffect, which arises owing to, inter alia, increased amounts of carbondioxide discharged to the atmosphere, is aimed at.

OBJECT OF THE INVENTION

The object of the present invention is to provide a method for producingelectric energy of high efficiency in a fuel cell by using a fuel whichis environmentally acceptable, easily accessible and safe.

This object is achieved by means of a method of the type mentioned byway of introduction and having the features recited in thecharacterising clause of the appended claims.

SUMMARY OF THE INVENTION

According to the present invention, a method for producing electricenergy in a biofuel-powered fuel cell is provided, in which method fuelis supplied to a first reactor 1 in the negative pole compartment of thefuel cell, in which said fuel is, under the action of a first catalyst2, reacted with a first acid metallic salt solution for completeoxidation of the fuel to carbon dioxide and reduction of the metal;

the CO₂ formed is discharged from said first reactor 1;

said first acid salt solution of reduced metal is passed to an anode 9in an electrochemical cell 14 for oxidising the metal;

said first acid metallic salt solution is subsequently recirculated tothe reactor 1;

an oxidising agent is supplied to a second reactor 17 in the positivepole compartment of said fuel cell, in which the oxidising agent is,under the action of a second catalyst 18, reacted with a second acidmetallic salt solution for oxidising the metal therein;

the residual product formed is discharged from the reactor 17;

the second acid metallic salt solution is subsequently passed to acathode 11 in the electrochemical cell 14 for reducing the metal;

the second acid metallic salt solution is subsequently recirculated tothe second reactor 17, the anode 9 and the cathode 11 of theelectrochemical cell 14 being separated merely by a hydrogen ionpermeable membrane 13, through which hydrogen ions from the cathode 11are transferred to the anode 9 for producing electric energy, saidmethod being characterised in that the metal in said first acid metallicsalt solution forms a redox pair having a normal potential between -0.1and 0.7 V, and that the metal in said second acid metallic salt solutionforms a redox pair having a normal potential between 0.7 and 1.3 V, bothmetals preferably being vanadium, which forms the redox pairsvanadium(IV)/(III) and vanadium(V)/(IV), respectively;

that carbohydrate is supplied as fuel to said first reactor 1, and

that the reaction in said first reactor 1 is effected in the presence ofplatinum or ruthenium as the first catalyst 2.

According to the present invention, the energy content of thecarbohydrate, more specifically the sugar, is utilised to a far higherdegree than has been possible with previous fuel cells, in which glucoseis used as fuel. A definitely higher efficiency is achieved, especiallybecause of the utilisation of vanadium in the solution of electrolytesof the fuel cell. Compared to the use of sugar as raw material forproducing ethanol, and then the use of the ethanol in an internalcombustion engine, the present method must be preferred, since a higherdegree of efficiency is achieved. This depends on the one hand on ahigher theoretical efficiency of the fuel cell as compared to a heatengine and, on the other hand--when impelling an electric vehicle--ahigher efficiency of the electric motor. The advantages of usingbiofuels, such as carbohydrates, are that in general the supply thereofis rich, and that they are environmentally relatively acceptable andquite nonpoisonous as fuels. Moreover, sugars which are not fermentableto ethanol may be used. Although, according to the present invention,carbon dioxide forms in the complete combustion of the carbohydrates,this occurs in an amount which is approximately as large as is requiredby nature for reproducing carbohydrates. From the viewpoint of Swedishagricultural policy, the present invention confers great advantagessince it leads to maintaining the agriculture developed in centuries andsafeguarding the type of landscape that is appreciated by all of us.

The voltage generated in the battery used for the present invention isD.C. voltage. This opens a range of application for the electrochemicalindustry, for e.g. the chlorine/alkali production. In this context,power losses owing to transformation and rectification are prevented.The fuel can be supplied to the electrochemical industry locally andthere be converted into electric power.

DESCRIPTION OF THE DRAWING

The invention will now be described in more detail with reference to theaccompanying drawing which essentially corresponds to the fuel cell asdescribed by J. T. Kummer and D. G. Oei, J. Appl. Electrochem. 12 (1982)87 and 15 (1985) 619, respectively.

The drawing concerns a fuel cell for producing electric energy. For thepurpose of simplification, the fuel cell can be considered to be dividedinto two compartments. In the same compartment as the negative pole ofthe fuel cell, i.e. to the right in the drawing, there is a firstreactor 1 containing a catalyst 2. A fuel conduit 3 for supplying fuelextends to the reactor 1. From the reactor 1 extends an outlet duct 4for discharging any undesired reaction product, and a duct 5 forconveying a solution of electrolytes to a first container 6. Thesolution of electrolytes can be conveyed from the container 6 by meansof pump 7 through a conduit 8 to the negative pole of an electrochemicalcell 14. The electrochemical cell 14 is divided into two compartments bymeans of a membrane 13. A first cell compartment 10 holds an anode 9,and a second cell compartment 12 holds a cathode 11. The two cellcompartments 10 and 12 and, consequently, also the anode 10 and thecathode 11 are thus separated by the membrane 13. From the first cellcompartment 10 extends a conduit 15 for returning the solution ofelectrolytes to the first reactor 1. From the second cell compartment12, which is positioned in the same half as the positive pole of thefuel cell, extends a conduit 16 for returning the solution ofelectrolytes to a second reactor 17, which comprises a second catalyst18. From the second reactor 17 extend an outlet duct 19 and a duct 20for conveying the solution of electrolytes to a container 21. To thesecond reactor 17 extends an inlet duct 24 for supplying oxygen. Bymeans of a pump 22, the solution can be passed via a duct 23 back to thesecond cell compartment 12. The fuel cell thus contains two separatecirculation systems, one in each half, in which the two circulatingsolutions of electrolytes are separated merely by the membrane 13 in theelectrochemical cell 14.

DESCRIPTION OF EMBODIMENTS

The carbohydrate used as biofuel may comprise monosaccharides such asglucose, fructose, disaccharides such as maltose, saccharose (canesugar, beet sugar), and penroses such as xylose and arabinose. Thecarbohydrate may also comprise polysaccharides which can be hydrolysedto monosaccharides, for example cellulose, starch and amylose. Alsomolasses or a hydrolysis product thereof is included. Preferably, use ismade of common sugar, i.e. saccharose having the formula C₁₂ H₂₂ O₁₁,glucose having the formula C₆ H₁₂ O₆, and xylose having the formula C₅H₁₀ O₅. Also other monosaccharides, oligosaccharides and hydrolysisproducts of polysaccharides, not enumerated here, are included in theterm carbohydrate.

Preferably, common sugar is supplied to the reactor 1. The sugar may beadded in solid form, also oligo- and polysaccharides, but may also bedissolved in advance in a suitable solution. The concentration of sugarin the reactor 1 should be in the range of about 0.001-0.1M, preferablyabout 0.003M. The first catalyst 2 in the first reactor 1 is made of aprecious metal, preferably ruthenium or platinum, most advantageouslyplatinum. Its object is to completely oxidise sugar according to thereaction

    C.sub.12 H.sub.22 O.sub.11 +13H.sub.2 O→12CO.sub.2 +48H.sup.+ +48e.sup.-

The first catalyst 2 is present in a finely divided state, eitherseparately or dispersed on a carrier. As carrier material for theCatalyst metal, use is preferably made of carbon, silicon dioxide,titanium dioxide, zirconium dioxide or some other material that does notdissolve in the acid solution of electrolytes and at the relatively hightemperature. Moreover, the catalyst is adapted not to leave the firstreactor 1 together with the circulating solution of electrolytes. Thesugar supplied to the first reactor 1 is dissolved in and reacts withthe solution of electrolytes, which is an acid metallic salt solution.The acid metallic salt solution consists of a salt of a metal dissolvedin a strong acid. The strong acid is preferably sulphuric acid orphosphoric acid, most advantageously sulphuric acid, and has aconcentration of about 0.05 to about 5.0M. The acid metallic saltsolution has a pH of -0.7 to 1.0. Also other acids which are notvolatile and do not oxidise are useful. The concentration of sugar,metallic salt and acid in the solution is mutually related and varieswithin the limits stated.

The metal in the acid metallic salt solution should be such as to form aredox pair with favourable properties for the method according to thepresent invention. The demand on the metallic salt system used at thenegative pole of the fuel cell is that the normal potential for theredox pair involved is greater than -0.1 V and less than 0.7 V. Themetal which has been found to be the most suitable one according to thepresent invention is vanadium, but also molybdenum and some ironcomplexes can be used. The metallic salt preferably used isvanadium(IV)sulphate, VOSO₄, since it is easily accessible and has beenfound to be suitable for this purpose. Also V₂ O₅ can be used, but notas successfully.

In the presence of the first catalyst 2, the dissolved sugar reacts withthe acid solution containing vanadium sulphate in which vanadium has theoxidation number +IV, whereby the vanadium is reduced so as to have theoxidation number +III. Besides, the vanadium can to a certain extent bereduced to the oxidation state +II, but the +III form is predominant.The total reaction occurring in the first reactor 1 is as follows:

    C.sub.12 H.sub.22 O.sub.11 +48VO.sup.2+ +48H.sup.+ →12CO.sub.2 +48V.sup.3+ 35H.sub.2 O

The carbon dioxide formed in the reaction is discharged through theoutlet duct 4 to the open. After the reaction in the first reactor 1,the reduced metallic salt solution is passed to the first container 6,from which it is pumped by means of the pump 7 through the conduit 8 tothe electrochemical cell 14. The reduced metallic salt solution ispassed specifically into the first cell compartment 10 at the negativepole of the electrochemical cell 14. The electrochemical cell 14 issurrounded by a cell casing of polypropylene. The reduced metallic saltsolution reaches in the first cell compartment 10 the anode 9, whichpreferably is a porous pressed felt of graphite. Connected to the anode9, there is at each end an electric switch. This also applies to thecathode 11. Under the action of the electrode, the metal in the metallicsalt solution is oxidised to a higher oxidation number, i.e. vanadiumobtains once more the oxidation number +IV. Subsequently, the oxidisedmetallic salt solution is discharged from the first cell compartment 10and recirculated through the conduit 15 back to the first reactor 1,whereupon the above-described procedure in this compartment of the fuelcell is repeated.

In the second compartment of the fuel cell, i.e. in the half in whichthe positive pole of the electrochemical cell 14 is to be found, anoxidising agent, such as air or pure oxygen, is supplied to the secondreactor 17 through the duct 24. Here, oxygen reacts, under the action ofthe second catalyst 18, with the solution of electrolytes, which is asecond acid metallic salt solution and which can differ from or be thesame as the acid metallic salt solution leaving the first cellcompartment 10 in the electrochemical cell 14 in the other circulationsystem as described above. Preferably, the acid metallic salt solutionshave identical properties in both cell compartments. Like in the othercirculation system, the acid is preferably sulphuric acid or phosphoricacid, most advantageously sulphuric acid, and may vary within the samepH limits. For the metal in the metallic salt solution in thiscompartment of the fuel cell, i.e. the positive pole compartment, thesame conditions apply as in the other compartment described above. Thesecond catalyst 18 is a solid substance which is practically insolublein acid and which catalyses the oxidation of the metallic salt to ahigher oxidation state under the action of the oxygen in the air.Preferably, vanadium is oxidised from the oxidation number +IV to theoxidation number +V, but also other suitable metallic salts can beoxidised from a lower to a higher oxidation number, e.g. molybdenum andiron. Instead of air, pure oxygen can be injected. The demand on themetallic salt system, which is used at the positive pole, is that thenormal potential of the redox pair involved in this case is less than1.3 V and greater than 0.7 V. The normal potential of vanadium in thepositive pole compartment is 1.0 V and in the negative pole compartment0.34 V in the fuel cell. The second,catalyst 18 can be a catalyst ofconventional type, which is used in the technique for effecting theelectrode reaction at the positive pole in a conventional fuel cell, butit may also be a different chemical system, for example HNO₃ /NO (see J.T. Kummer and D. G. Oei, J. Appl. Electrochem. 15 (1985) 619).

In the second reactor 17, the reaction, when using vanadium as metal, isas follows:

    O.sub.2 +4VO.sup.2+ +2H.sub.2 O→4VO.sub.2.sup.+ +4H.sup.+

Thus, vanadium is oxidised to a higher oxidation number (+V), whereuponthe acid metallic salt solution oxidised in the reaction in the secondreactor 17 passes from the same through the duct 20 to the secondcontainer 21, from which it is pumped by means of the pump 22 throughthe duct 23 to the second cell compartment 12 in the electrochemicalcell 14. Under the action of the cathode 11, the metal in the acidmetallic salt solution is reduced to a lower oxidation number (+IV),whereupon the solution is recirculated through the conduit 16 back tothe second reactor 17 for repeating the procedure as described above atthe positive pole of the fuel cell. From the second reactor 17, thenitrogen gas formed, if air is used as oxidising agent, is discharged tothe open air through the duct 19.

According to the present invention, the metal in the acid metallic saltsolution in the two circulation systems of the fuel cells is, asmentioned above, preferably the same substance, vanadium beingespecially preferred. The redox pairs concerned are then V(V)/V(IV) andV(IV)/V(III), respectively. In the compartment of the fuel cell, whichrepresents the negative pole, the metallic salt is thus presentessentially in a lower oxidation state, and in the cell compartmentrepresenting the positive pole in the fuel cell, the metallic salt ispresent essentially with a higher oxidation number. The voltage betweenthe two electrodes, i.e. the anode 9 and the cathode 11, in theelectrochemical cell 14 is dependent on how well this balance between alow and a high oxidation number can be maintained. The current takenfrom the fuel cell according to the present invention is dependent onthe concentration of low- and high-oxidised metal ions in the circulatedmetallic salt solutions. The two metallic salts should be such as not toallow any undesired type of reaction by a possible diffusion or flow ofmetallic salt from one to the other cell compartment, i.e. between thecell compartments 10 and 12. The membrane 13, which separates the twocirculation systems, is designed so as to permit proton transport only.The above-mentioned undesired reactions can be such as to result inprecipitation of at least one of the active salts, a coating on themembrane so as to change its pores, or effects on the potential or thepotential adjusting speed at the respective electrode.

The electrodes, i.e. the anode 9 and the cathode 11, can be made ofmetal or some other conductive material and be designed such that a highconversion of the reduced and, respectively, oxidised metallic saltoccurs at the surface. The highest possible exchange current perexternal surface of the electrode at issue is thus aimed at. Accordingto the present invention, use is made of a porous electrode surface.Moreover, additives to the electrode material may be used to increasethe exchange current. These additives can be either precious metals orso-called redox systems, whose normal potential is close to that of theredox system of the metallic salts. The used membrane 13 is made of, forexample, polyethylene or polypropylene, whose pores are filled with asuitable material permitting proton transport, while preventing thesolutions from flowing freely between the respective cell compartments.An example of filling materials is silica gel or some other inorganic,amphoteric material which is insoluble in acid.

When using the fuel cell according to the present invention, atemperature of preferably max. 150° C., most advantageously about90°-105° C., is used continuously in the fuel cell.

It is thus the unique combination of carbohydrate as fuel, vanadium inacid solution and platinum as the first catalyst 2 in a fuel cell, whichpermits production of electric energy with a high degree of efficiencyaccording to the present invention.

EXAMPLES

The invention can be used for producing electric energy on a large orsmall scale. The example below concerns a small "power plant" whichshould produce a power of 10 kW. Such a power plant comprises aplurality of components, i.e. the battery of individual cells yieldingthe electric energy, and the reactors in which the chemical reactionsoccur. Moreover, auxiliary equipment, such as pumps and control devices,are included.

In a preferred example, common cane sugar (sacharose) is supplied to thefirst reactor 1. The acid metallic salt solution is vanadium(IV)sulphatedissolved in phosphoric acid. The phosphoric acid concentration is 5M (5moles/liter=490 g phosphoric acid/liter) and the sugar concentration is0.003M (1.03 g sugar/liter). By using platinum having a specific surfaceof 100 m² /g as the first catalyst 2, a sufficiently high chemicalreaction rate is obtained for producing the desired power at a vanadiumsulphate concentration of 0.3M (49 g VOSO₄ /liter).

The obtained chemical reaction rate at a temperature for the firstreactor 1 of 100° C. is 3 moles vanadium/h. This means that 489 g (3×163g) VOSO₄ has been reduced by the sugar during 1 h. According toFaraday's law, which is known to those skilled in the art, this chemicalreaction rate can be converted into current intensity, i.e. theconverted quantity of electricity per time unit. In this example, thecurrent intensity is 78 A/liter reaction solution when using 1.25 gplatinum catalyst. To obtain a power of 10 kW, a fuel cell can beprovided according to the enclosed drawing so as to produce a cellvoltage of 0.7 V when the cell yields so much current that the load persurface unit of the electrodes is 15 mA/cm². At this voltage, a quantityof current of 14,285 A (10,000 W/0.7 V) must be produced. The volume ofthe first reactor 1, in which the sugar reacts, must thus be 183 liters(14,285/78) and contain a catalyst amount of 229 g platinum (183×1.25).

The second reactor 17 for oxidising vanadium(IV) to vanadium(V) is ofapproximately the same size as the first reactor 1. In the secondreactor 17, use is also made of a solid or soluble catalyst 18 having aredox potential in the range of 0.8-1.2, e.g. a phthalocyaninepreparation of a suitable metal, preferably cobalt, or nitrogendioxide/nitric acid.

The electrochemical cell according to the invention preferably has acell voltage of about 0.7 V. For obtaining a useful electric voltage, alarge number of individual cells must be connected in series. A suitablevoltage, which is used in e.g. experiments with electrically drivenvehicles, is 90 V. In this example, this voltage is used, and then thenumber of cells connected in series is 130. In this example, 4 units of130 cells connected in series have been connected in parallel. Thesurface of the electrodes in each unit is 18.5 dm² (for example 4×4.6dm), which yields a total electrode surface for the entire battery of 74dm². The above-mentioned current density of 15 mA/cm² results in acurrent intensity of 111 A (15×74×100 mA). The product of cell voltageand current intensity will then be 10 kW (90 V×111 A).

In a preferred example, the electrochemical cell 14 is enclosed by a3-mm-thick cell casing. The space between the cell casing and thecathode 11 or anode 9 is 0.5 mm. Both electrodes have a thickness of 2mm, and the fluid layer between electrode and membrane is 0.5 mm thick.The membrane is 1 mm thick and the electrode surface is 18.5 dm² (4×4.6dm). The same dimensions apply to both halves in the electrochemicalcell 14. This means a total thickness of the electrochemical cell of 13mm. 130 cells in series then yield a minimum length of 17 dm (130×0.13dm). The volume of one unit will then be 313 liters (17 dm×19 dm²), andthe total volume will be 1250 liters (4×313 liters). This gives acurrent density of 10 kW/1250 liters, i.e. 8 W/liter. The theoreticalcell voltage of a sugar fuel cell is 1.25 V. The electric efficiencywill thus be 56% (0.7/1.25).

We claim:
 1. A method for producing electric energy in a biofuel-poweredfuel cell, comprisingsupplying carbohydrate fuel to a first reactor in anegative pole compartment of the fuel cell containing a precious metalas a first catalyst, reacting said fuel with a first acid metallic saltsolution for complete oxidation of the fuel to carbon dioxide andreduction of the metal; discharging said carbon dioxide formed from saidfirst reactor; passing said acid salt solution containing reduced metalto an anode in an electrochemical cell to oxidize the metal andsubsequently recirculating it to the first reactor; supplying anoxidizing agent to a second reactor in a positive pole compartment ofsaid fuel cell containing a second catalyst to react with a second acidmetallic salt solution to oxidize the metal therein; discharging aresidual product formed from said second reactor; passing the oxidizedsecond acid metallic salt solution to a cathode in an electrochemicalcell to reduce the metal; recirculating the second acid metallic saltsolution to the second reactor; wherein the anode and the cathode of theelectrochemical cell is separated by a hydrogen ion permeable membranethrough which hydrogen ions from the cathode are transferred to theanode for producing electric energy, and wherein the metal in said firstacid metallic salt solution forms a redox pair having a normal potentialbetween -0.1 and 0.7 V, and the metal in said second acid metallic saltsolution forms a redox pair having a normal potential between 0.7 and1.3 V, wherein the redox pair formed is Vanadium (iv)/(iii) or Vanadium(v)/(iv).
 2. The method according to claim 1, wherein the carbohydratefuel is in the form of saccharose, glucose, maltose, xylose and/orarabinose, or cellulose, starch or amylose, or a hydrolysis product ofthese carbohydrates, or molasses or a hydrolysis product thereof and issupplied as fuel to said first reactor in a concentration of up to about0.1M.
 3. The method according to claim 2, wherein said concentration is0.003M.
 4. The method according to claim 1, wherein the acid in saidfirst and second acid metallic salt solution is sulphuric acid orphosphoric acid in a concentration of about 0.05 to about 5M.
 5. Themethod according to claim 1, wherein said second catalyst has a redoxpotential in the range of 0.8-1.2.
 6. The method according to claim 1,wherein the temperature in the fuel cell is a maximum 150° C.
 7. Themethod according to claim 6, wherein the temperature is 90°-105° C. 8.The method according to claim 1, wherein vanadium in the form ofvanadium(IV) sulphate is reacted in said first reactor and/or in saidsecond reactor.
 9. The method according to claim 1, wherein a salt ofmolybdenum, tungsten or iron in said solution is reacted in said firstreactor and/or in said second reactor.
 10. The method according to claim1, wherein vanadium in the negative pole compartment of said fuel cellis also reduced to the oxidation state +II.
 11. The method according toclaim 1, wherein said second is nitric oxide/nitric acid.
 12. The methodaccording to claim 1, wherein said precious metal is Pt or Ru.
 13. Themethod according to claim 12, wherein said precious metal is Pt.